Circuit for measuring impredance in the heart

ABSTRACT

A circuit for determining the impedance in a heart having a pacemaker electrode to which voltages are applied disposed in the ventricle chamber thereof. The circuit uses the pacemaker electrode. The circuit includes a current source operatively connected to the pacemaker electrode to apply current thereto, the current source having a current direction which can be switched. Switching circuitry is provided for switching the current direction of the current source. The voltage applied to the pacemaker electrode at different measuring times is measured with measuring circuitry. A value representative of the impedance in the heart is determined from a difference between the voltages applied to the pacemaker electrode at two of the different measuring times measured by the measuring circuitry. Currents having an identical magnitude but having opposite directions are applied to the pacemaker electrode by the current source which is switched by the switching circuitry at the two respective measuring times.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an arrangement for for determining theimpedance in the heart by way of the pacemaker electrode disposed in theventricle chamber.

2. Background Information

In some cases it is favorable to determine the electrical impedance inthe heart region, using measuring technology, during operation ofpacemakers, particularly in connection with an adaptation of thestimulation rate to the instantaneous stress on the patient.

It is known, for example, to determine the cardiac stroke volume in themanner of impedance plethysmography, i.e., by means of a measurement ofthe impedance in the heart. To do this, in the simplest case twoelectrodes are inserted into the heart. If a current of a knownmagnitude flows through the cardiac volume via these electrodes, theimpedance in the heart can be determined by means of a voltagemeasurement. A corresponding apparatus for measuring the ventricularvolume is described in DE-36 29 587-A1. This type of arrangement is notsuitable for use in pacemakers.

From EP-A-0 140 472, a pacemaker is known in which the impedance in theheart is measured by way of further electrodes disposed at the pacemakerelectrode, and the stimulation rate is controlled with the stroke volumedetermined from this. In this case, a series of measurements is providedfrom whose average value the stroke volume is calculated, which however,has disadvantages.

SUMMARY OF THE INVENTION

The object of the invention is to disclose an arrangement of the generictype mentioned at the outset, which is particularly suited for use inpacemakers.

This object is accomplished with the characterizing features of theinvention, as will be described below.

The invention is based on the recognition that, in impedancemeasurements in the heart which ultimately always amount to combinedcurrent/voltage measurements, it is important to eliminate, if possible,the interference voltages occurring there, of which, among other things,a polarization voltage stemming from the stimulation pulse can be apart. This must take place with the smallest possible energy expenditurebecause of the limited battery charge.

In the circuit according to the invention, the pacemaker electrode isconnected to a current source that can be switched in its currentdirection and be controlled with respect to its current intensity. Thevoltage applied to the pacemaker electrode is measured by at least onevoltage measurement apparatus.

Measurement is particularly effected outside of the times of thestimulation pulses or the times that must be available for detection ofthe signals emitted by the heart.

If a voltage not stemming from the current flow is present at thepacemaker electrode, this voltage makes a corresponding contribution tothe measured voltage, i.e., this voltage measurement is erroneous. Thismeasuring error is eliminated by the reversal of the current directionby means of corresponding switching means, and the remeasurement of thevoltage applied to the pacemaker electrode. In this measurement, thevoltage not stemming from the current flow makes the same contributionas in the first measurement. In the subtraction of the two measuredvoltages, these contributions cancel each other out, while thenon-erroneous components are added, because their operational signs arereversed by the current reversal.

The currents necessary for measurement need not be made so large in thisway that the voltage dropoff caused by them, which takes place at theimpedance, is significantly greater than the polarization voltage. Thisleads to an increased service life of the pacemaker battery.

In an advantageous modification of the invention, the magnitude of thecurrent delivered by the current source and flowing by way of thepacemaker electrode is controlled as a function of the differencebetween the voltages measured consecutively in the currentless state. Inthis way the intensity of the current can be optimally adapted to theinterference level, which cannot be compensated, leading to a furtherrelief of the pacemaker battery.

In an advantageous manner, the delivery of current by the current sourceis effected in the form of pulse packets, with each pulse packetcomprising two sequential rectangular pulses. This combines numerousadvantages: on the one hand, the energy requirement for measurementdecreases significantly; on the other hand, in an impedance that changestemporally in this manner, which is the case of the beating heart, theinstantaneous values of the impedance can be measured. These values aremore precise than those obtained through long averaging, because theexternal voltage not caused by the measuring current only changesslightly in pulses that follow one another directly.

In another advantageous modification of the invention, the individualmeasurements, i.e., the delivery of the pulse packets, are effected atlarge time intervals with respect to the time interval between theindividual pulses. This limitation to few pulse packets is particularlyadvantageous in connection with the current intensity, which can becontrolled as a function of the interference level. The interferenceinterval increases in linear fashion with the increase in the current,that is, with the current consumption, whereas, in contrast, instatistical averaging the interference interval only increases with theroot from the number of measurements, and thus only with the root of thecurrent consumption. This is of greater significance in implantablepacemakers, because a battery change is always associated with anoperation.

It is often not necessary to know the absolute value of the impedance inthe heart; rather, information about its relative change is sufficient.Such relative measurements are to be performed with significantlygreater precision than an absolute measurement, especially when thesignals to be measured are very small. In such cases, the measuringcircuits themselves could produce errors.

In the circuit of the invention, therefore, first a voltage that isrepresentative of the temporal change in the impedance in the heart isformed by the subtraction from a voltage that is measured at a certainpoint in time, and is representative of the impedance, of a furthervoltage that is measured at a later point in time; both of thesevoltages are obtained through measurements in pulse packets thatcoincide in their pulse sequence. Erroneous voltages that are possiblyadded by the circuit itself are eliminated with this measure.

In a further step, the difference voltage obtained in this manner isadded with a further difference voltage. This further difference voltageis obtained as above, but by means of voltage measurements of pulsepackets having an opposite pulse sequence. In this process, first avoltage having the opposite operational sign is also measured, whichsign is reversed again by switching means having an intensity that canbe controlled with respect to their operational sign.

Erroneous voltages that are a function of operational sign are noweliminated by the summation of the two difference voltages.

Now a sum of temporal changes in impedance in the heart appears at theoutput of the circuit; the average temporal change can be determinedfrom this sum through the formation of an average value.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantageous modifications of the invention are set forth in detailbelow, together with the description of the preferred embodiment of theinvention, by way of the figures. Shown are in:

FIG. 1 a block diagram of the embodiment of the invention;

FIG. 2 a block diagram of an input stage of the embodiment according toFIG. 1;

FIG. 3 a portion of the embodiment according to FIGS. 1 and 2;

FIG. 4 a block diagram of a portion of the embodiment of the inventionaccording to FIGS. 1 through 3;

FIG. 5 a block diagram of a current source according to FIG. 1

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the basic design of the circuit according to the inventionin a block diagram. The impedance in the heart to be determined isrepresented as a resistor 101 having a voltage source 102 connected inseries. The voltage source 102 symbolizes the polarization voltagestemming from the pacemaker pulse and other interference voltages. Theimpedance is charged with a current from a current source 103. Thevoltage applied to the impedance is detected with a scanning block 104.Its output signal is conducted further to a first difference amplifier105, whose output signal is determined from the difference between twovoltages that were measured at different times by the scanningfunctional block 104. Moreover, the scanning block 104 is connected to acontrol 106 which controls the intensity of the current produced by thecurrent source 103 as a function of the instantaneous interferencevoltage level.

The signal coming from the difference amplifier 105 is conducted furtherto a second difference amplifier 107. This amplifier forms thedifference between two difference voltages of different operationalsigns. The output signal of the difference amplifier 107 is the outputsignal of the circuit.

FIG. 2 shows a block diagram of an input stage of the scanningfunctional block 104. The voltage applied to the impedance in the heartis applied by way of a controllable switch 201 to a capacitor 202 thatis connected to ground by way of a further controllable switch. Thecapacitor is charged by the voltage applied to the impedance.

After a certain period of time, the switches 201 and 203 open, whereupontwo further controllable switches 204 and 205 close and a capacitor 206is charged.

In a further step, the switch 204 opens. Afterwards a controllableswitch 207 is closed, together with a further controllable switch 208.The difference between the voltages of the capacitors 206 and 202 is nowapplied to the input of an operational amplifier 209. A capacitor 210and a further controllable switch 211 lie in its negative feedbackbranch.

The operational amplifier, which is wired as an integrator, compensatesfor the losses occurring because of unavoidable leakage currents of thecapacitors 202 and 206, so that a signal that is proportional to thedifference of the voltages which are applied to the impedance andmeasured at different times appears at the output of the input stage.

If these voltages are measured at opposing currents through theimpedance, the interference voltages occurring in both cases, which haveidentical operational signs and are independent of the currents,mutually compensate each other.

The pulses necessary to control the switches are obtained from the clockgenerator provided in the pacemaker.

FIG. 3 shows a portion of the circuit. Two circuit blocks 301 and 302are located at the input. Each of these blocks represents an input stageof the above-described type. At different times, these blocks deliveroutput voltages that are representative of the impedance in the heart;these voltages are fed to an amplifier 303.

A capacitor 306 is charged with the amplified signal of the input signal301 via the switches 304 and 305. With the aid of further switches 307and 308, the voltage of the capacitor 306 is then applied with areversed operational sign to the input of a further amplifier 309. Theamplified signal of the input stage 302 is again fed to the amplifier309, with the same operational sign, by way of the switches 304 and 308via the capacitor 306, so that the difference between the amplifiedvoltages of the input stages 301 and 302 appears at its output.

FIG. 4 shows a block diagram of the circuit having the scanning unit104, the difference amplifier 105 and the circuit 107 shown in detail.The difference voltage of two scanning units is applied to a capacitor401. In the process, a switch 402 is closed. With the aid of a furtherswitch 403, the voltage of the capacitor 401 serves as a reference for adifference voltage of two further input stages of the scanning unit, thevoltage having the opposite operational sign. The sum of the twodifference voltages appears at the output of an operational amplifier404, in whose reverse feedback branch a capacitor 405 and a switch 406lie.

FIG. 5 shows the block diagram of a current source of the embodiment.For clarification of the current flow, the impedance 101 to be measuredis also represented.

In the resting state, i.e., without current delivery via the impedance101, a switch 501 is closed. Further switches 502, 503, 504 and 505 areopen. Two resistors 506 and 507 maintain two capacitors 508 and 509 athalf of the operational voltage.

In order to deliver a positive current pulse via the impedance 101, theswitch 501 is opened and the switches 503 and 505 are closed. Thevoltage of the capacitor 509 now lies in series with the positiveoperating voltage, so that the capacitor discharges via the impedance.The magnitude of the flowing current is determined by a current source510.

In a negative current pulse, the switches 502 and 504 are closed, sothat the capacitor 508 is charged via the impedance 101. Because thecurrent source 510 again lies in series with the impedance 101 and thecapacitor 508, it also determines the flowing current here.

The invention is not limited in its configuration to the preferredembodiment described above. Rather, a number of variations areconceivable which also make use of the represented solution in othertypes of embodiments.

What is claimed is:
 1. A circuit for determining the impedance in aheart, a pacemaker electrode to which voltages are applied beingdisposed in the ventricle chamber of the heart, the circuit using thepacemaker electrode, the circuit comprising:a current source operativelyconnected to the pacemaker electrode to apply current thereto, thecurrent source having a current direction which can be switched;switching means for switching the current direction of the currentsource; measuring means for measuring the voltage applied to thepacemaker electrode at different measuring times; and determining meansfor determining a value representative of the impedance in the heart bydetermining a difference between the voltages applied to the pacemakerelectrode at two of the different measuring times measured by themeasuring means; wherein currents having an identical magnitude buthaving opposite directions are applied to the pacemaker electrode by thecurrent source which is switched by the switching means so that at thetwo respective measuring times the respective currents applied haveopposite directions.
 2. The circuit according to claim 1, furthercomprising control means for controlling the magnitude of the currentdelivered by the current source and flowing by way of the pacemakerelectrode as a function of the magnitude of the difference between thevoltages applied to the pacemaker electrode at the different measuringtimes.
 3. The circuit according to claim 1, wherein the current sourceproduces a current comprising pulse packets, wherein each of the pulsepackets includes two rectangular pulses of opposite polarity that followone another in close succession.
 4. The circuit according to claim 3,wherein a time interval between the pulse packets is relatively largecompared with a temporal spacing between individual rectangular pulseswithin one of the pulse packets.
 5. The circuit according to claim 3,wherein the sequence of the individual rectangular pulses in asuccessive pulse packet is opposite to the sequence of individualrectangular pulses in a preceding pulse packet, whereby if a pulsepacket has a sequence which is positive-negative, a successive pulsepacket will have a sequence which is negative-positive.
 6. The circuitaccording to claim 3, wherein the determining means further comprisescircuit means for amplifying the voltage representative of the impedanceso that the polarity thereof remains unchanged in each of the pulsepackets.
 7. Circuit according to claim 3, wherein, with the determiningmeans, a difference voltage that is representative of a temporal changein impedance in the heart is formed by subtracting from a first voltagemeasured at a first point in time representative of the impedance at thefirst point in time, a second voltage measured at a second later pointin time, and wherein both the first and second voltages are obtainedthrough measurements in pulse packets that coincide in their pulsesequence.
 8. The circuit according to claim 7, further comprising meansfor forming a sum from two difference voltages representative of thetemporal change, wherein the two difference voltages are respectivelyobtained through measurements in pulse packet pairs that differ in theirpulse sequence.